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Physics 1 > Attenuation > Flashcards

Attenuation Flashcards

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1
Q

attenuation

A

the weaking of the sound beam as it travels

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2
Q

clinical importance of attenuation (3)

A

limits our imaging depth
must be compensated for (TGC/gain)
can be useful in diagnosis

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3
Q

bel/decibel are what

A

relative units of measurement expressing the loudness (intensity level) of sound waves

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4
Q

1 bel (B) = ____ decibels (dB)

A

10

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5
Q

0 dB as a percentage for US

A

100%

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6
Q

instrument ____ uses decibels to determine the ____ of the sound leaving the transducer

A

output
power

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7
Q

dynamic range used decibels to express the ______ of shades of grey displayed on a monitor

A

number

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8
Q

gain and TGC use decibels to express the amount of ______ or ______ required to optimize the returning echoes

A

amplification
attenuation

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9
Q

formula to calculated bels/dB

A

b=log(new I / original I)

dB=10log (new I / original I)

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10
Q

power formula

A

dB=10log(new P / original P)

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11
Q

voltage formula

A

dB=20log(new V / old V)

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12
Q

1/2 original intensity = ___ dB drop

A

3

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13
Q

.1 of og intensity= ___ dB drop

A

10

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14
Q

10db drop =

A

.1 of og intensity

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15
Q

3dB drop=

A

1/2 original intensity

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16
Q

input/output power = what in power formula

A

input= Po (original P)
output= Pn (new P)

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17
Q

attenuation coefficient

A

amount of attenuation that occurs with each one cm travelled

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18
Q

attenuation coefficient in soft tissue

A

.5 dB for every one cm travelled per 1 MHz

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19
Q

attenuation coefficient in soft tissue is equivalent to _____

A

1/2 freqeuncy

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20
Q

Total attenuation formula

A

Tatt= att. coef. x path length (cm)

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21
Q

total attenuation formula for soft tissure

A

Tatt= 1/2f x path length (cm)

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22
Q

half-value layer is what

A

the distance sound must travel in a material to reduce the intensity to half its original value

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23
Q

attenuation can vary with the following (3)

A

nature of tissue (dead/alive)
frequency of the US
depth

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24
Q

half-value layer formula

A

Tatt=1/2f x path

but

3dB loss= 1/2 og intensity

so

formula=
3dB=1/2f x path

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25
Q

5 reasons for attenuation

A

absorption
reflection
refraction
scatter (kinda a part of reflection)
wave-front divergence

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26
Q

absorption (what, special, accounts for)

A

the conversion of sound energy to heat and is the dominant factor in attenuation (accounts for about 80%)

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27
Q

3 things affecting absorption and what is dominant

A

viscosity
relaxation time on molecules
frequency (dominant)

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28
Q

what is viscosity

A

thickness - ability of molecules to slide past one another

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29
Q

increased viscosity = _____ resistance, ______ friction, and thus, _____ attenuation

A

greater
more
increased

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30
Q

what is friction (and what part of attenuation it accounts for)

A

how sound energy is converted to heat

absorption

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31
Q

relaxation time (and what part of attenuation it accounts for)

A

time it takes vibrating molecules to come to rest

absorption

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32
Q

if molecules cant come to rest before the next compression phase, than ___ energy is require to ____ its direction. This produces ____

A

more
reverse
heat

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33
Q

increase relaxation time= ____ absorption

A

increased

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34
Q

how does frequency affect absorption

A

F increased = less time available for molecules to recover during relaxation process = more absorption

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35
Q

two types of reflection

A

specular
non-specular

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36
Q

specular reflection occurs when

A

a sound beam hits a large, smooth surface

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37
Q

what is a large surface relative to

A

to the frequency used and is greater than one wavelength in diameter

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38
Q

good specular reflector examples

A

diaphragm, portal vein

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39
Q

intensity of sound reflected relies on two things

A

angle of incidence
acoustic impedance of two media

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40
Q

angle of incidence equals the angle of

A

relfection

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41
Q

reflections that have non-perpendicular incidence may …

A

not return to probe

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42
Q

________ incidence helps improve relfection

A

perpendicular

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43
Q

acoustic impedance AKA/what (formula)

A

character impedance

describes the relationship between acoustic pressure and the speed of particle vibrations in a sound wave
Impedance (Z) = density (p) x velocity (c)

44
Q

how fast a sound moves through a medium depends on mediums ______ (propagation speed)

A

density and stiffness

45
Q

impedance formula

A

impedance (Z) = density (p) x velocity (c)

46
Q

unit of impedance

A

Rayls (Z)

47
Q

Z increases with ______ density or velocity

A

increases

48
Q

acoustic impedance ____ depend on frequency

A

does not

49
Q

the _____ the Z value difference at the interface of 2 media the bigger the reflection

A

larger

Rayls (Z) = unit for impedance

50
Q

if Z values are the same =

A

no relfection

Rayls (Z) = unit for impedance

51
Q

intensity reflection coefficient

A

amount of sound that reflects at an interface

52
Q

intensity reflection coefficient formula

A

IRC = reflected intensity (Ir) / incident intensity (Ii)

or

IRC = (Z2 - Z1) / (Z2 + Z1) all squared

53
Q

IRC answers will always be

A

less than one (multiply by 100 for percentage)

54
Q

intensity transmission coefficient formula

A

ITC = 1 - IRC

55
Q

what is the intensity transmission coefficient

A

amount of sound transmitted

56
Q

more sound reflected = _____ sound transmitted

A

less

57
Q

reflection occurs because of a difference in ____values across an interface

A

Z (Rayls)

58
Q

it ___ possible for 2 tissues to have different sound velocities but the same Z values

A

is

59
Q

reflection vs refraction

A

sound returning to the probe from an interface
vs
sound that is transmitted across the interface

60
Q

Snell’s law of optics

A

Sin θi / Sin θt = Vi / Vt

V=velocity
i= incidence
t= transmitted

61
Q

sound can bend across an interface due to a difference in media ____

A

veolcity

62
Q

velocities across interface are equal =

A

no refraction

63
Q

velocity of first medium is greater than that of second (V1>V2) =

A

sound bends towards normal (new angle less than what you started with)

64
Q

velocity of of first medium is less than the second (V1<V2) =

A

sound bends away from normal (larger angle than what you started with)

65
Q

total internal reflection

A

occurs when V1 is less than V2 and the angle reaches a critical value where it is straight with the interface

66
Q

refraction _____ relate to Z values

A

does not

67
Q

scatter is the result of

A

sound interacting with interfaces smaller than a wavelength and rough

68
Q

scatter is typically from ____ media, and is _____ of sound direction

A

heterogenous

independent

69
Q

scatter is responsible for the internal _____ of organs

A

texture

70
Q

scatter results in the incident sound beam ____

A

breaking up into many different echoes each with its own intensities and direction

71
Q

amount of scatter depends on (2)

A

frequency: higher F = more scatter
reflector size: smaller = more scatter

72
Q

acoustic speckle

A

phenomenon where the interference patterns from echoes undergo multi-path scattering resulting in potential for brightness non-uniformities

73
Q

phenomenon where the interference patterns from echoes undergo multi-path scattering resulting in potential for brightness non-uniformities

A

acoustic speckle

74
Q

Rayleigh scatter

A

specific to RBC’s

RBC’s are way smaller than the wavelength of sound = very weak scatter = we don’t see blood flow in vessels at faster velocities (can sometimes see in IVC where slower/clumps near heart)

75
Q

as the sound beam diverges, the intensity of the beam _____ and adds to the overall ____ of the beam

A

weakens
attenuation

76
Q

Media that measure less than one wavelength and contribute to the parenchyma or fine texture pattern of an image

A

non specular reflectors

77
Q

The dominant factor in attenuation in which the sound energy is converted to heat

A

absorption

78
Q

The redirection of sound in many directions and intensities by rough surfaces or heterogeneous media measuring less than one wavelength

A

scatter

79
Q

The relative unit of measurement that expresses the intensity or loudness of sound

A

bel

80
Q

The throwing back of sound without absorbing it as the result of a difference in acoustic impedances across an interface

A

reflection

81
Q

One tenth of a Bel

A

decibel

82
Q

large smooth surfaces, measuring greater than one wavelength, that return relatively large amounts of sound to the source

A

specular reflectors

83
Q

The weakening of sound as it travels

A

attenuation

84
Q

The ability of the molecules of a medium to slide past one another

A

viscosity

85
Q

The amount of time it takes for the molecules in a medium to come to rest after a mechanical force is applied

A

relaxation time

86
Q

The bending of sound across an interface when the angle of incidence is non-perpendicular and the velocities of sound differ in the two media

A

refraction

87
Q

The amount of tissue required to attenuate sound to half of its original intensity

A

half value layer

88
Q

The amount decibels lost per centimeter of travel

A

attenuation coefficient

89
Q

The spreading out of the sound beam as it travels

A

divergence

90
Q

What type of reflectors will result in scatter?

A) Specular reflectors
B) Non-specular reflectors
C) Reflectors that are smooth
D) Reflectors that are greater than 1 wavelength

A

b

91
Q

What happens to the amount of absorption if there is an increase in relaxation time?

A) It increases
B) It decreases
C) It stays the same
D) Relaxation time does not effect absorption

A

a

92
Q

What term describes an image that has non-uniform areas of brightness as the result of multipath echoes interfering when they reach the probe?

A) Refraction
B) Acoustic speckle
C) Destructive interference
D) Constructive interference

A

b

93
Q

If the frequency of the sound emitted is increased then what is true of the total attenuation under the same scanning conditions?

A) It increases
B) It decreases
C) It is unchanged
D) Frequency has no impact on total attenuation

A

a

94
Q

Which of the following conditions may result in total internal reflection of sound at an interface?

A) Velocity 2 is less than velocity 1 with perpendicular incidence
B) Velocity 2 is greater than velocity 1 with perpendicular incidence
C) Velocity 2 is less than velocity 1 with non-perpendicular incidence
D) Velocity 2 is greater than velocity 1 with non-perpendicular incidence

A

d

95
Q

Sound has an intensity of 30 dB and is attenuated to 27 dB after traveling 3 cm in soft tissue. What is true of this distance?

A) It is the half value layer
B) It reduced the power by half
C) It reduced the intensity by half
D) All of the above are correct

A

d

96
Q

How many decibels per centimeter are attenuated for every 1 MHz of frequency in soft tissue?

A) 0.3 dB
B) 0.5 dB
C) 0.7 dB
D) 1.0 dB

A

b

97
Q

What is the most dominant factor in attenuation?

A) Scatter
B) Reflection
C) Refraction
D) Absorption

A

d

98
Q

Which of the following is needed for reflection of sound to occur at an interface?

A) Z values must differ
B) Densities must differ
C) Velocities must differ
D) Z values must be equal

A

a

99
Q

name all things affected when increasing frequency emitted from probe

A

Decrease: wavelength, T, PD, SPL, DF
increase: res, absorption. scatter

100
Q

tools/ scanning procedures that can compensate for attenuation (and what problems it helps

A

decrease F (absorption, scatter)
get perpendicular (reflection, refraction, scatter)
adjust divergence (wave front divergence)
increase power,
change window
use gel
TGC/Gain

101
Q

what values relates the spatial peak intensity to the spatial average intensity and how

A

BUR= SP/SA

102
Q

what factor has the greatest influence on the velocity of sound

A

compressibility

103
Q

what type of reflector is the parenchyma of the body/why

A

non-specular/scatter as smaller than a wavelength

104
Q

what is the reduced attenuation of tissues seen deep to anechoic structures called

A

posterior/acoustic enhancement
through transmission

105
Q

how does Huygen’s principle affect transducers sensitibity

A

con/destructive interference from wavelets give natural focus, manipulate focus = changes sensitivity

106
Q

what is it called when two waves are neither completely in or out of phase

A

beat frequency

107
Q

When scanning from tissue 1, with an acoustic velocity of 1550 m/s, to tissue 2 that has an acoustic velocity of 1530 m/s, there is no reflection at the interface. How could this be possible?

A

Reflection depends on Z values and not velocities. It is possible that the tissues have equal Z values and different velocities.

Physics 1 (7 decks)

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